The present invention relates to pumps, valves and analogous fluid conveying or fluid flow controlling devices, and more particularly to improvements in stuffing boxes which can be used in or in combination with such devices to prevent the leakage of conveyed fluid media along the peripheral surfaces of rotary and/or reciprocating members. Still more particularly, the invention relates to improvements in stuffing boxes which are especially suited to prevent leakage of fluid media between the body or housing and the rotating shaft (movable member) of a centrifugal pump, for example, a centrifugal pump which is designed to convey fluid media at elevated temperatures, media which contain solid particles, media which are subjected to a very pronounced pressurizing action and/or media which contain toxic or other deleterious ingredients.
The shaft seal of a pump for circulation of a hot fluid medium which is maintained at an elevated pressure and/or contains toxic or other deleterious ingredients as well as solid particles must be designed with a view to positively prevent leakage of any, or to positively prevent leakage of appreciable quantities of, the fluid medium which passes through the pump housing. Heretofore known shaft seals (primarily stuffing boxes) are incapable of satisfying all of the above requirements, i.e., if the stuffing box can stand elevated pressures and/or temperatures, it cannot withstand the corrosive action of solids which are dispersed in the fluid and/or the action of other constituents including acids or the like. For example, water or another liquid which is used for the scrubbing of gases often contains several aggressive ingredients as well as tar which presents additional problems.
When the gland of the stuffing box is biased against the packing by a pressurized sealing fluid other than the fluid medium which is circulated in the housing of a centrifugal pump, the pressure of sealing fluid must exceed the pressure of fluid medium in the pump housing; therefore, the highly pressurized sealing fluid exhibits a strong tendency to escape into the surrounding atmosphere. The rate at which the sealing fluid leaks increases as the wear upon the sealing means for preventing or reducing such leakage increases. Therefore, presently known stuffing boxes which operate with sealing fluid must be equipped with complex, bulky and expensive auxiliary components which insure that the pressure of sealing fluid does not drop below the pressure of fluid medium in the pump housing in spite of progressively increasing leakage of sealing fluid into the surrounding atmosphere. In the absence of such auxiliary equipment, the pressure of sealing fluid rapidly drops below the pressure in the interior of the pump housing so that the conveyed medium penetrates into the stuffing box and mixes with the sealing fluid. Consequently, the sealing fluid which escapes from the stuffing box contains at least traces of the conveyed fluid medium which is especially undesirable when the fluid medium is very hot and/or contains toxic or other deleterious ingredients.
Attempts to prevent excessive leakage of sealing fluid from the stuffing box include the provision of double-acting friction seals. A double-acting friction seal is likely to control the leakage of sealing fluid for a certain interval of time; however its useful life is short, especially if the pressure of sealing fluid is relatively high (i.e., higher than the pressure in the interior of the pump housing). In order to insure relatively long useful life of a double-acting friction seal, it is necessary to assemble such seal from components which are made of expensive material. Moreover, the nature of the sealing fluid cannot be selected at will, i.e., the sealing fluid should not attack the constituents of the double-acting seal. All in all, the cost of a stuffing box which employs a pressurized sealing fluid and one or more double-acting friction seals is extremely high which is particularly undesirable because the useful life of seals for the sealing fluid is rather short, even if such seals are made of an expensive material and are machined or otherwise shaped with a high degree of precision.
A double-acting friction seal is especially affected by highly viscous constituents of the fluid medium in the pump body. For example, if the fluid medium contains tar which escapes into the stuffing box and is admixed to the sealing fluid, and if the viscosity of tar increases in response to cooling of the sealing fluid, the hardened tar prevents slippage of the constituents of a double-acting friction seal with respect to the shaft of a pump or with respect to a rotary element of a valve.
Another serious drawback of double-acting friction seals is that, when the pressure of sealing fluid drops (i.e., as a result of malfunction of the apparatus which pressurizes the sealing fluid), the double-acting seal cannot prevent immediate penetration of the fluid medium from the pump housing into the stuffing box.